Abstract : A semi-empirical formula which correctly predicts most of the properties of observed missile exhaust trail expansions has been developed. The general agreement between formula and observation lends credence to the belief that the combined hydrodynamical-diffusion processes from which the formula is developed contain the essential mechanisms involved in the trail expansion. That this agreement is obtained in spite of the complete neglect of the initial velocities do not play an essential role in the expansion phenomena. Qualititative considerations of the chemiluminescent time constants associated with the problem show that these are probably of the order of or greater than the equivalent dynamical time constants. Hence, once the dynamical problem is solved, it is probably not necessary to solve additional equations to obtain the radiation density. The formulas also predict definite radiation intensities, but these cannot be calculated without a knowledge of the chemiluminescent reaction constant and the fractions of exhaust and ambient gases which are active in producing the radiation. (Author)